This invention relates generally to solar cells and more particularly to a semiconductor solar cell including a p-n junction and having means for minimizing surface effects such as surface recombination and resistance.
To understand the invention clearly, it is first useful to describe prior art devices. In FIG. 1, there is shown a prior art solar cell which includes a heavily doped n-type (n+) emitter fabricated on a lightly doped p-type (p-) body. The cell has a heavily doped p-type (p+) back contact region and a graded p-type layer extending from the back contact into the cell interior or body. The impurity concentration of the cell is shown schematically in FIG. 2. This graded p-type layer produces an electric field E.sub.BSF at the back surface directed in such a way as to force electrons in the graded region back into the body of the cell. Hence, electrons that are either generated in the graded region by the incident light or diffused into the graded region from the cell body are forced back into the body of the cell rather than being permitted to travel to the p+ contact. Since the electron lifetime is very low in the p+ contact region whereas the electron lifetime is higher in the p- body the back surface field redirects the electrons near the p+ contact from regions of low lifetime into regions of high lifetime. This then increases the effective lifetime of electrons and, therefore, increases the probability of collecting them in a suitably placed n-type contact on the surface.
In cells of the type shown in FIG. 1, the n+ contact is made by diffusing or ion implanting an n+ layer on the surface of the cell that is exposed to light. In order to be collected and contribute to the output current, electrons generated in the body of the cell must diffuse to the n+ layer. The back surface field aids in this process by keeping the electrons away from the p+ contact where electron recombination is high. However, diffusion transport through the body of the cell is still required for electrons to reach the n+ layer and electron lifetime in the body of the cell, while larger than the electron lifetime near the p+ contact, may still be sufficiently low that many electrons recombine before they can diffuse to the n+ layer.
The problem of electron recombination in the cell body can be minimized by using an extended drift field E.sub.EDF in the body of the cell. An extended drift field device is illustrated in FIG. 3. The device includes a graded p-type doped region which extends substantially across the body of the cell. The extended p-type doping gradient produces the electric field E.sub.EDF which causes electrons to be transported efficiently from the body to the n+ layer despite low carrier lifetime in the cell body. The impurity concentration of the cell is shown schematically in FIG. 4. Reference is made to my U.S. Pat. No. 4,001,864 which describes solar cells including extended electric fields at the p-n junction cell for extracting the photo-generated carriers. The back surface electric field E.sub.BSF minimizes the deleterious effects of low electron lifetime near the p+ contact and the extended drift field E.sub.EDF in the body minimizes the effect of low electron lifetime in the cell of the body.
However, even with the introduction of these drift fields, there exists surface effects at the n+ surface of the device which reduce the efficiency and introduce series resistance. Holes generated in the n+ layer must be transported efficiently to the p+ contact. The electric fields E.sub.EDF and E.sub.BSF generated by the doping gradients just mentioned provide efficient transport to the p+ contact for holes that are generated in these regions. But holes that are generated in the n+ layer are in a region where the hole lifetime is low and hence holes recombine quickly in the n+ layer. This effect can be minimized by making the n+ layer relatively thin. However, deleterious effects also arise when the n+ layer is made too thin. The problem is then that electrons reaching the n+ layer from the cell body must flow along the n+ layer to the external contacts of the device. This lateral flow of electrons in the n+ layer introduces resistance into the device. This internal resistance absorbs part of the power that is generated in the device and, therefore, reduces the cell power output. A heavily doped n+ layer will reduce the internal resistance but at the same time not permit holes to diffuse to the cell body because hole lifetime in the n+ contact is lower with higher doping concentration. A more lightly doped n+ layer, or a graded n+ layer, can be used, but then the lateral resistance of the layer increases. Hence, the requirements of low cell resistance and efficient collection of both types of carriers are in conflict at the front surface of the prior art devices.